A resin film, for example, a biaxially stretched polyester film, has been widely used as a base film of optical members such as touch panel, liquid crystal display and plasma display. An example of the optical members in which the resin film is used is a hard coat film attached to a front surface of the display. The hard coat film is used such that hard coat films having scratch-resistant, wear-resistant properties, and the like are multilayered on one surface of the resin film. The hard coat films having such properties are constituted by, for example, acrylic resin, urethane resin, melamine resin, epoxy resin, or polyester resin and have preferably a film thickness of 1 to 15 μm.
As a conventional method for obtaining the multilayered hard coat films, the surface of the resin film is coated with a coating liquid obtained by diluting monomer or oligomer of active-ray-curing resin with an organic solvent by means of the die coating method, rod coating method, roll coating method, gravure coating method or the like, the organic solvent is thereafter evaporated in a drying oven, and then, the surface of the resin film is irradiated by active ray such as ultraviolet ray and cured. A large volume of organic solvent is used in the conventional method, however, there are problems not only of imposing a heavy burden on environments but also of increasing manufacturing costs since the method requires a coating/drying apparatus provided with an explosion-proof equipment.
In order to deal with the disadvantage, as disclosed in Patent Document 1, the hard coat film exerting the scratch-resistant and wear-resistant properties and the like was invented, wherein a coating liquid containing acrylic oligomer and reactant diluent as its main components was applied during a resin film manufacturing process. Such coating liquid is advantageous in that the impact on the environments can be reduced because the amount of the organic solvent to be contained can be reduced to such an extent that safety can be guaranteed when the coating liquid is applied in a conventional resin film manufacturing apparatus not provided with the explosion-proof equipment. Further, since a coating step (referred to as off-line coating), which has been conventionally carried but in a coating-only line apart from a manufacturing line for the resin film, is unnecessary, the manufacturing costs of the hard coat film can be reduced. However, the conventional coating liquid diluted with the organic solvent has the viscosity in the range of approximately 1 to 10 mPa·s, while the viscosity of the coating liquid in which the amount of the organic solvent is minimized is at least 200 mPa·s. Thus, the proposed coating liquid has newly generated a problem of the significant deterioration of a coating performance.
In the generally called in-line coating wherein the coating liquid is applied during the resin film manufacturing process, an oven in an apparatus called a tenter is conventionally used so that the applied coating liquid is dried. The tenter is an apparatus which nips both ends of the resin film with a clip and laterally stretches or simultaneously biaxially stretches the resin film. The clip nips the resin film intermittently, which generates vibrations in the resin film. It is known that the vibrations are transmitted to a coating unit, which generates a coating unevenness called transverse thickness-difference lines (for example, see Patent Document 2).
Therefore, the rod coating method is widely used in the in-line coating. In a known example of the rod coating method, for example, as disclosed in Patent Document 3, a coating rod is pressed onto a lower surface of the resin film in a state where the coating rod is circumscribed and supported by support members each comprising a pair of rollers and spaced with intervals therebetween in a length direction of the coating rod, and the excess coating liquid supplied to the resin film in advance is scraped off (measured). Such rod-coating method makes it difficult for the transverse thickness-difference lines to occur because a peak of a groove formed on an outer peripheral surface of the coating rod contacts the resin film, which suppresses the vibrations of the resin film. The phrase “a peak of a groove contacts the resin film” denotes a state where the peak of the groove is basically in direct contact with the resin film. In a practical sense, however, a very thin film derived from the coating liquid may be interposed between the peak of the groove and the resin film, which is also included in the contact state described above.
The rod coating method is suitably used for the in-line coating because an apparatus has a simple configuration and is easily operable and maintainable and coating can be applied at a relatively high speed. However, the range of the viscosity of the coating liquid suitable for the rod coating method is usually at most 100 mPa·s (for example, Non-Patent Document 1).
One of the reasons why it is difficult to apply the coating liquid having a high viscosity in the rod coating method is that the coating liquid having a high viscosity has a low fluidity and it is thus difficult to collect and reuse the coating liquid scraped off by the coating rod. Even if the coating liquid could be collected by means such as suction, a special apparatus and an extra processing time would be required in order to remove air bubbles and foreign matters included in the coating liquid, which would deteriorate the productivity. As an example of the rod coating method in which the coating liquid is applied without being reused, there is a known method as disclosed in Patent Document 4, wherein the coating liquid is supplied from the support members of the coating rod so that an amount of the coating liquid picked up by the coating rod and an amount of the coating liquid applied to the resin film can be equal to each other. In this method, however, the viscosity of the coating liquid is preferably at most 100 mPa·s. If the coating liquid has a high viscosity, the coating liquid is streaked when it is picked up from the support members by the coating rod, and a liquid pool formed immediately before portions of the coating rod and the resin film contacting each other is unevenly generated in a width direction of the resin film. Thus, a coating defect called coating streaks is easily generated.
As an example of the rod coating method in which the coating liquid is applied without being reused such that the liquid pool formed immediately before the portions of the coating rod and the resin film contacting each other is evenly generated, there is a known method as disclosed in Patent Document 5, wherein the coating liquid is supplied to an upper surface of the resin film by a die arranged in the vicinity of the coating rod. According to this method, the liquid pool formed immediately before the portions of the coating rod and the resin film contacting each other is spread by the action of gravity, and thereby evenly spread in the width direction of the resin film even when the coating liquid have a high viscosity. However, when the coating liquid passes through a space surrounded by the coating rod and the resin film (that is, the groove formed on the outer peripheral surface of the coating rod), the space between the coating rod and the resin film is subjected to a high pressure in the case where the coating liquid has a high viscosity because a pressure generated in the coating liquid is proportional to the viscosity of the coating liquid. In the conventional in-line coating, the resin film coated with the coating liquid, both ends of which are hipped by the clip of the tenter, is easily deflected in the width direction thereof. If the viscosity of the coating liquid is high, therefore, the coating rod and the resin film are separated from each other at the center of the resin film by the pressure of the coating liquid, which causes such problems that a coating thickness may be larger than desired, and the coating thickness may be uneven in the width direction of the resin film.
There is a conventionally known method for preventing the deflection of the resin film in the width direction thereof (for example, see Patent Documents 6 and 7), wherein the upper surface of the resin film is pressed by guide rolls provided before and forth the coating rod when the lower surface of the resin film is coated with the coating liquid by the coating rod so that a constant winding angle of the resin film relative to the coating rod is obtained. Also in the method, however, the viscosity of the coating liquid is preferably at most 100 mPa·s due to the problems described above, such as the collection of the coating liquid and the generation of streaks when the coating liquid is picked up by the coating rod, because the lower surface of the resin film is coated with the coating liquid. Further, the coating liquid cannot be applied to both surfaces of the resin film at the same time.
Examples of the coating method suitable for applying the coating liquid having the viscosity of at least 200 mPa·s, which are known, are the die coating method, roll coating method, gravure coating method and the like. According to these methods, a coating tool (die, coating roll, gravure roll) and the resin film are kept from contacting each other, and the coating liquid retained in a gap between the coating tool and the resin film is applied. Therefore, if these methods are applied to the in-line coating, the vibrations of the resin film caused by the tenter described above induce the vibrations of the coating liquid retained in the gap between the coating tool and the resin film, which easily results in the generation of the transverse thickness-difference lines. Further, an apparatus used therein has a complicate structure in comparison to the rod coating method, and an operation and maintenance of the apparatus are also complicated.    PATENT DOCUMENT 1 Japanese Patent Applications Laid-Open No. 2005-041205    PATENT DOCUMENT 2 Japanese Patent Applications Laid-Open No. 2004-174410    PATENT DOCUMENT 3 Japanese Patent Applications Laid-Open No. 2001-276713    PATENT DOCUMENT 4 Japanese Patent No. 1169863    PATENT DOCUMENT 5 Japanese Patent No. 2805177    PATENT DOCUMENT 6 Japanese Patent Applications Laid-Open No. 2000-107661    PATENT DOCUMENT 7 Japanese Patent Applications Laid-Open No. 2004-113963    NON-PATENT DOCUMENT 1 Page 56, “Coating Methods” by Yuji Harasaki, Japan, published by Maki Shoten on Oct. 30, 1979.